Prevent '\b' from backing up into invalid memory.  Fixes http://llvm.org/bugs/show_bug.cgi?id=16240.  Sorry, I can not think of a good test case for this one, except by running valgrind as reported in the bug.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185273 91177308-0d34-0410-b5e6-96231b3b80d8